Green tapes are typically prepared by slip casting a slurry of the ceramic powder onto a nonporous carrier such as a glass or stainless steel plate or a plastic film of polypropylene or Mylar.RTM. (glycol terephthalic acid polyester). A doctor blade or knife is passed over the slurry-coated carrier to achieve the desired slip thickness for the green tape. The volatile constituents in the slurry are removed by evaporation or other drying processes. The resulting green tape, if properly formulated, can easily be removed from the carrier. The green tapes which are fabricated into the multilevel structure are finally fired to remove the binder and other residual materials remaining from the drying step and to sinter the ceramic powder into a continuous ceramic dielectric structure.
In the fabrication of such structures, one of the chief problems has been that the prior art binders have not been sufficiently removed during the firing step at temperatures which maintain the ceramic in a porous condition during such a step. These binders have included cellulosic resins such as cellulose nitrates, cellulose esters, alkyl cellulose ethers, hydroxyalkylcellulose ethers, alkyl hydroxyalkylcellulose ethers and dialkylene glycol cellulose ethers and polymers such as polyurethane, polyvinylbutyral, polyvinyl acetate, poly(alphamethylstyrene), poly(methylmethacrylate), polyisobutylene, poly(oxymethylene), polyethylene and polypropylene.
U.S. Pat. No. 3,780,150 (1973) discloses the basic prior art method for making thin alumina ceramics from green tapes in which alumina is milled with an azeotropic solvent mixture of trichloroethylene and alcohol, talc and a wetting agent or surfactant to a desired surface area per gram of dielectric; a slip is formulated of the dielectric mixture with the same solvent mixture, a suitable binder, e.g. polyvinylbutyral, and a mixture of triethylene glycol hexanoate and phthlate esters as the plasticizer; de-airing and casting the slip on a substrate; removing the volatiles; separating the resulting green tape from the substrate; and firing the green tape at about 1425.degree. to 1550.degree. C.
U.S. Pat. No. 3,899,554 (1975) discloses a method for making multilayered ceramic structures from a green tape formulated by using a ceramic powder, a thermoplastic binder resin system, e.g. polyvinylbutyral polymer and dioctyl phthalate plasticizer, dissolved in a volatile solvent mixture comprising both a solvent and a non-solvent for the resin. The preferred solvent mixture is an azeotrope of methanol and toluene.
U.S. Pat. Nos. 4,080,414 (1978) and 4,104,345 (1978) disclose a process for forming ceramic substrates from a green tape formulated by using a ceramic powder such as alumina, the same binder resin system described above dissolved in a volatile solvent mixture in an amount so that the Brookfield viscosity for the resulting slurry composition is about 500 to about 2000 cps. The preferred solvent mixture comprises methanol and methyl isobutyl ketone where the ratio of the evaporation rate of methanol to methyl isobutyl ketone is at least 2.
U.S. Pat. No. 4.234,367 (1980) discloses a method for making multilayered glass-ceramic structures with copper based metallurgy from green tapes using a thermoplastic binder resin system, e.g. polyvinylbutyral polymer and a dioctyl phthalate or dibutyl phthalate plasticizer. A pattern of the copper based conductor is formed on a first green tape and a second green tape is superimposed on the first to sandwich the pattern therebetween. A laminating press is used to laminate the superimposed tapes which is heated in the presence of hydrogen and H.sub.2 O to burn-out the binder at lower binder removal temperatures than is ordinarily used with such binder systems. However, the temperatures for burn out are still in the range of 720.degree. to 785.degree. C.
U.S. Pat. No. 4,413,061 (1983) discloses a method for making multilayered glass-ceramic structures from green tapes using a thermoplastic binder resin system, e.g. polyvinylbutyral polymer and a dibutyl phthalate plasticizer. The glass ceramic comprises a mixture of the .beta.-spodumene type and alphacordierite. The sintering takes place at temperatures in the range of 870.degree. to 1000.degree. C.
U.S. Pat. No. 4,474,731 (1984) discloses a process for making green tapes in which a compact of ceramic powder, a polymeric hydrocarbon binder such as Butvar B-98, which comprises 80 mole % polyvinyl butyral, 18-20 mole % polyvinyl alcohol and up to 2.5 mole % polyvinyl acetate, in which nickel or palladium ions are dissolved in the binder to serve as a catalyst in the firing step at temperatures of 350.degree. to 780.degree. C.
U.S. Pat. No. 4,504,339 (1985) discloses a method for making green tapes using a copper-based conductor, a thermally depolymerizable resin such as poly(alphamethylstyrene), poly(methylmethacrylate) or polytetrafluoroethylene, and firing the resulting green tape under inert atmospheric conditions containing water vapor to overcome the problem of binder removal during firing.
U.S. Pat. No. 4,540,621 (1985) discloses a method for making multilayered glass-ceramic structures from green tapes using crystalline cordierite which has a coefficient of thermal expansion of about 15.times.10.sup.-7 .degree. C..sup.-1 at 20.degree. to 100.degree. C. a binder such as Butvar B-98 referred to above. Prior to sintering at temperature of about 1300.degree. to 1450.degree. C., a molybdenum and/or tungsten pattern is deposited on the green tape.
U.S. Pat. No. 4,598 107 (1986) discloses a method for forming a slurry for casting into a ceramic green tape using a thermoplastic organic binder such as polyvinylbutyral, a plasticizer such as dipropylene glycol dibenzoate, an organic solvent, and a ceramic powder consisting of alumina and glass frit. The slurry is formed from a pre-mix of the solvent, plasticizer, and binder having a relatively low viscosity of 2-8 cps, mixing the glass frit into the pre-mix and dispersing the alumina into the mixture to form an intermediate slurry and finally combining the latter with a high viscosity post-mix of solvent and binder having a relatively high viscosity of 3,000-30,000 cps.
U.S Pat. No. 4,627,160 (1986) discloses a method for forming a glass-ceramic composite substrate in which a catalyst of a copper-containing material is incorporated into the slurry to accelerate the oxidation of the binder and to eliminate carbonaceous binder residue. The binder is described as being any suitable one such as polyvinylbutyral resin.
U.S. Pat. No. 4,752,857 (1988) discloses a method for making green tapes using the cellulosic resins as binders, a dielectric component and a solvent to solubilize the binder.
U.S. Pat. No. 4,766,027 (1988) discloses a method for overcoming the problem of binder burn-out by incorporating internal copper conductors into the slurry used to make the green tapes. The green tapes were made, for example, by combining a thick copper paste of butyl methacrylate resin dissolved in terpineol, and copper powder with a ceramic dielectric material, e.g. a nonreducing glass dispersed in an acrylic polymer binder. Upon firing the resulting green tape, most of the organic binder is removed by preheating in nitrogen at 400.degree. C. for about an hour.
"Nitrogen-Nitrous Oxide A Reactive Atmosphere for Copper Thick-Film Processing", E. A. Hayduk Jr..and B. M. Adams, International Society for Hybrid Manufacturing Proceedings of 1987 Conference of ISHM, pages 569-576 teaches the use of nitrous oxide reactive gas atmospheres which are effective in burning out binders from copper multilayer green tape circuitry.
Japanese Patent Application SHO 62/21753 (1987) and its European Patent Office counterpart EP-300039-A (1989) discloses using poly(alkylene carbonate), preferably poly(ethylene carbonate) and poly(propylene carbonate), as binders for molding ceramic or metallic powders. Additives such as plasticizers, lubricants, wetting agents. surfactants and other additives and other binders, are disclosed as being used as long as the purpose is not adversely affected.
U.S. Pat. Nos. 4.814,370 (1989) and 4,882,110 (1989) teach the use of CO.sub.2 copolymers such as poly(propylene carbonate) as binders for ceramic bodies which are shaped into a suitable green body using conventional procedures, e.g. extrusion molding, injection molding, tape shaping, compression molding, slip casting and the like. Examples are given in which green compacts of alumnia and poly(propylene carbonate) are more effectively sintered in air at 1550.degree. C. for 2 hours than other binders such as polyvinyl alcohol and methylcellulose binders.
U.S. Pat. No. 4,874,030 (1989) discloses compositions useful in decomposition molding procedures such as destructive foam casting or ceramic or metallic powder sintering which comprise blends of polymers having greater than 50 weight percent propylene carbonate units and polymers having greater than 50 weight percent methyl methacrylate units. Preferred blends contain 35 to 65 weight percent poly(methyl methacrylate) and 65 to 35 weight percent poly(propylene carbonate).
U.S. Pat. Nos. 3,585,168 (1971) and 3,953,383 (1976) describe methods for preparing poly(propylene carbonate) by copolymerizing propylene oxide and carbon dioxide. A more detailed description of the preparation of poly(propylene carbonate) resins is given in Inoue, Higashi and Yamazaki, "Synthesis of Macromolecules from Carbon Dioxide", Organic and Bio-Organic Chemistry of Carbon Dioxide, Chapter 4, John Wiley & Sons, New York (1982). The description for the preparation of such resins is incorporated herein by reference.
It has been found that conventional prior art procedures are not sufficient to produce satisfactory green tapes using poly(propylene carbonate) as a binder. There is nothing in any of the prior art references which indicates the unexpectedly low temperatures, in either an oxidizing or a non-oxidizing atmosphere, that can be used for firing dry green tapes containing poly(propylene carbonate), ceramic powder and plasticizer in order to remove substantially all of the binder system as determined by thermogravimetric anaylsis (TGA).